Hi guys, 
i need to repair a mould which has some damage to the gelcoat, i was wondering if the process is just simply putting some tooling gelcoat into the damaged area and wait for it to dry before sanding it down?

is there any powder substance i can use to make the gelcoat into a 'filler' ?

In essence you are correct.  If it is a styrene based gel, then adding Solution MW as a wax additive is useful to give a hard non-tacky surface.  Although if the resin is proud anyway, you can sand through the tack as you level out the repair.  

Most gelcoats are fairly thixotropic anyway so no need to add a filler.  If the damage is deep into the fibreglass, you could repair that area with some chopped up fibres and resin prior to doing the last mm or so with the tooling gel.

Warren Penalver
Easy Composites / Carbon Mods - Technical Support Assistant

Solution MW Wax Gelcoat Additive is designed to work on styrene based resins that cure tacky in air such as most polyesters and vinylesters. To overcome this problem a Wax in Styrene, Solution MW Wax Additive, is added to unsaturated polyester and vinylester gelcoats at a low percentage (usually around 2%). Solution MW, which is made up of paraffin waxes in a styrene and white spirit solution, is pushed to the surface during curing and effectively forms a very thin barrier between the gelcoat and the air ensuring that the gelcoat cures with a hard, tack-free finish.

Warren Penalver
Easy Composites / Carbon Mods - Technical Support Assistant